The present invention relates generally to a metal oxide varistor (MOV) of the type having an integral thermally activated fuse for protection. More specifically, the present invention relates to an MOV device having a fuse that causes the varistor to go open circuit in conditions of overheating due to sustained over-voltages.
One such device is described in U.S. Pat. No. 5,901,027 (Leviton), in which a flat thermal fusible layer is deposited on a MOV element. U.S. Pat. No. 5,708,553 (Hung) also describes such a varistor, in which a lead is spaced-apart from an electrode and is connected to it by a column of solder extending outwardly from the electrode.
While these varistors appear to be reasonably effective, there is scope for improving characteristics in such a device. One such improvement is provision of an improved insulation gap after fusing, without relying on properties such as outgassing in an epoxy. Another desirable improvement is better handling of transient peak currents. It is also desirable that manufacturing be simplified.
There is a need for an improved varistor device that provides integrated thermal protection.
To this end, in an embodiment, the present invention provides a thermally protected metal oxide varistor including a varistor body, a number of electrodes on the varistor body one of the electrodes being a fused electrode, a number of leads, and a fuse connecting a lead to the fused electrode. The fuse includes an insulator overlying part of the fused electrode; a link having a portion overlying the insulator and a portion electrically connected to the fused electrode, the link being of a material having a melting point at or below a thermal safety temperature threshold for the varistor; and a body of hot melt material in contact with the link, the hot melt material being an electrical insulator and having a melting point such that it melts and flows to create an insulating gap between the fused electrode and the material of the link when the link becomes molten.
In an embodiment, the link is of elongate wire shape.
In an embodiment, the link includes a solder material and internal flux within the solder material.
In an embodiment, the solder material is Sn/Pb.
In an embodiment, the flux is located centrally within the link.
In an embodiment, the hot melt material surrounds the link between the insulator and the fused electrode.
In an embodiment, the hot melt material is in contact with the fused electrode.
In an embodiment, a portion of the hot melt material lies between the fused electrode and the link.
In an embodiment, the link is electrically connected to the fused electrode by a low temperature solder fillet.
In an embodiment, the hot melt material acts to retain the link in position, so that the link has a stable position before encapsulation.
In another embodiment, the present invention provides a metal oxide varistor including a varistor body, electrodes including a fused electrode, leads, and a thermal fuse connecting a lead to the fused electrode. The fuse includes an insulator overlying part of the fused electrode; and a link of elongate shape and including flux surrounded by solder material having a melting point at or below a thermal safety temperature threshold for the varistor. The link further includes a first portion in contact with the fused electrode, a second portion surrounded by a body of hot melt material, said hot melt material also being in contact with the fused electrode, and a third portion overlying the insulator and being connected to a lead. The hot melt material is an electrical insulator and has a melting point such that it melts and flows to create an insulating gap between the fused electrode and the material of the link when the link becomes molten.
Accordingly, it is an advantage of the invention to provide a varistor that has integrated thermal protection to protect against damage due to sustained over-voltages.
Other features and advantages of the present invention will be described in and are apparent from the detailed description of the presently preferred embodiments.